Beta-Decay Studies for Applied and Basic Nuclear Physics

EPJ manuscript No. (will be inserted by the editor) Beta-decay studies for applied and basic nuclear physics A. Algora1;2, J. L. Tain1, B. Rubio1, M. Fallot3, and W. Gelletly4 1 IFIC (CSIC-Univ. Valencia), Paterna, Spain 2 Institute of Nuclear Research (ATOMKI), Debrecen, Hungary 3 Subatech (CNRS/in2p3 - Univ. Nantes - IMTA), Nantes, France 4 University of Surrey, Surrey, UK Received: date / Revised version: date Abstract. In this review we will present the results of recent beta-decay studies using the total absorption technique that cover topics of interest for applications, nuclear structure and astrophysics. The decays studied were selected primarily because they have a large impact on the prediction of a) the decay heat in reactors, important for the safety of present and future reactors and b) the reactor electron anti- neutrino spectrum, of interest for particle/nuclear physics and reactor monitoring. For these studies the total absorption technique was chosen, since it is the only method that allows one to obtain beta decay probabilities free from a systematic error called the Pandemonium effect. The total absorption technique is based on the detection of the gamma cascades that follow the initial beta decay. For this reason the technique requires the use of calorimeters with very high gamma detection efficiency. The measurements presented and discussed here were performed mainly at the IGISOL facility of the University of Jyväskylä (Finland) using isotopically pure beams provided by the JYFLTRAP Penning trap. Examples are presented to show that the results of our measurements on selected nuclei have had a large impact on predictions of both the decay heat and the anti-neutrino spectrum from reactors. Some of the cases involve beta- delayed neutron emission thus one can study the competition between gamma- and neutron-emission from states above the neutron separation energy. The gamma-to-neutron emission ratios can be used to constrain neutron capture (n,γ) cross sections for unstable nuclei of interest in astrophysics. The information obtained from the measurements can also be used to test nuclear model predictions of half-lives and Pn values for decays of interest in astrophysical network calculations. These comparisons also provide insights into aspects of nuclear structure in particular regions of the nuclear chart. PACS. 21.10.Pc Single-particle levels and strength functions { 23.40.s β decay; double β decay; electron and muon capture { 26.50.+x Nuclear physics aspects of novae, supernovae, and other explosive environ- ments { 29.30.h Spectrometers and spectroscopic techniques { 29.90.+r Other topics in elementary-particle and nuclear physics experimental methods and instrumentation 1 Introduction states involved. Thus characteristic alpha and gamma ray spectra exhibit a series of discrete lines. It requires sophis- Our knowledge of the properties of atomic nuclei is derived ticated detection and analysis techniques to determine the almost entirely from studies of nuclear reactions and ra- excitation energies of the states involved, their lifetimes dioactive decays. The ground and excited states of nuclei and the transition rates between states. Beta decay carries arXiv:2007.07918v1 [nucl-ex] 15 Jul 2020 exhibit many forms of decay but the most common are al- the same information, but the difficulties of measurement pha, beta and gamma-ray emission. Our focus here is on and interpretation are compounded because the spectrum beta decay in its various manifestations. A glance at the is continuous, not discrete. In 1930 this was explained by Segre Chart reveals that it is the most common way for Pauli's hypothesis [1] of the existence of a neutral, zero the ground states of nuclei to decay and it is frequently mass particle called in his letter the neutron that is emit- the observation of such beta decays that brings us our first ted with the beta particle. The sharing of momentum and knowledge of a particular nuclear species and its proper- energy then explains the continuous spectrum. Shortly af- ties. terwards Fermi [2] was able to formulate a theory of beta The study of beta decay is intrinsically much more decay based on this idea and coined the name neutrino difficult than the study of either alpha or gamma decay. (little neutral one) for the particle. The reason for this is straightforward. Alpha particles and A knowledge of beta decay transition probabilities is gamma rays are emitted with discrete energies determined of particular importance for application to a) tests of nu- by the differences in energy between the initial and final clear model calculations, b) the radioactive decay heat in 2 A. Algora et al.: Beta-decay studies for applied and basic nuclear physics Fig. 2. Simplified picture of a beta decay where only one excited state is populated and it de-excites by the emission of a gamma cascade. The left hand panel represents the case. The Fig. 1. Schematic picture of how the beta feeding is deter- central panel presents the Pandemonium effect, in this example mined in a beta decay experiment employing Ge detectors. represented by missing, or not detecting the gamma transition The beta feeding (Iβ (i)) to level i is determined from the dif- γ2. The right hand panel represents the displacement of the ference of the total intensity feeding the level and those de- beta decay intensity because of the non detection of the tran- exciting it. The sum over (k) represents all transitions feeding sition γ2. or de-exciting the level. Iγk stands for the gamma intensity of transition k and ICEk represents the conversion electron intensity. means that we have a problem that has become known as the Pandemonium effect [3] (see Figure 2 for a simplified picture). reactors, c) the reactor electron anti-neutrino spectrum We can overcome this problem using the total absorp- and d) reaction network calculations for nucleosynthesis tion gamma spectroscopy technique, where we take a dif- in explosive stellar events. In this article we will provide ferent approach. The method involves a large 4π scintilla- examples of our recent studies of beta decays that involve tion detector and is based on the detection of the full de- the use of total absorption gamma spectroscopy (TAGS) excitation gamma cascade for each populated level, rather to tackle the topics listed above. The TAGS method was than the individual gamma rays. The power of TAGS to adopted in our measurements because it overcomes the find the missing beta intensity has been demonstrated in difficulties inherent in the conventional use of Ge detector a number of papers [4,5,6,7,8,9,10,11,12,13]. The use of arrays for this purpose. Such arrays are an important and the TAGS method began at ISOLDE[14]. Its development essential tool for constructing nuclear decay schemes since and history are described in [15,16]. they are very well suited to the study of gamma-gamma Looking at a wider picture we see that many entries coincidences, the main basis for building such schemes. in the international databases, that rely on measurements The normal practice is then to derive beta decay transi- with Ge detectors alone, will have systematic errors. As tion probabilities for each level populated from the differ- we shall see in the sections that follow this means that ence in the total intensity of all the gamma rays feeding the results cannot be relied on for certain applications. the level and the sum of the intensities of all those de- The answer to the resulting difficulties lies in the use of exciting it, corrected by the effect of internal conversion TAGS. In the remainder of this article we will describe (see Figure 1). In principle this allows us to obtain the the TAGS method in more detail and then use our results beta branching to every level, assuming that we are able to illustrate how it can be applied. to determine by some other means the number of decays The structure of this article is the following: in Section that go directly to the daughter ground state, which are 2 details of the experimental method and the analysis of not accompanied by gamma emission. the spectra are described. Sections 3, 4, 5 and 6 deal with Unfortunately this "simple" procedure does not neces- beta decay studies related to a) radioactive decay heat sarily give us the correct answers. States at high excita- (DH), b) reactor antineutrino spectra c) nuclear models, tion energies in the daughter nucleus can be populated if and d) astrophysical applications respectively. Finally, in the Q value of the decay is large. In this case both the β Section 7, a summary will be presented. number of levels that can be directly populated by the beta decay is large and the number of levels available to which they can gamma decay is also large. As a result, in general, individual gamma rays (emitted by levels at high 2 TAGS measurements excitation energy) have low intensity. Ge detectors, indeed even gamma-ray arrays, have limited detection efficiencies In Section 1 it was already explained why we need TAGS particularly at higher energies and thus weak transitions measurements. Figure 3 shows how the simple beta de- are often not detected in experiments. It is clear that this cay presented in Figure 2 is detected by typical detectors A. Algora et al.: Beta-decay studies for applied and basic nuclear physics 3 in bin i of the TAGS spectrum, fj is the beta feeding to the level j (our goal) and Ci is the contribution of the con- taminants to bin i of the TAGS spectrum. The index j in the sum runs over the levels populated in the daugther nucleus in the beta decay.

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